The present invention relates to multiple-channel frequency-modulation stereo systems. More particularly, it pertains to methods and apparatus for encoding and decoding multiple channel stereo signals.
Present-day broadcast FM stereo features the transmission of a two-channel coherent stereo signal the modulation function of which may be represented: EQU M' (t)= K' (L+R)+ K" (L-R)sin.omega. .sub.s t, (1)
where L represents a left-side audio signal, R represents a right-side audio signal, .omega..sub.s is the frequency of a suppressed carrier amplitude-modulated sub-carrier signal, t is time, and K' and K" are constants. A two-channel stereo receiver responds to a stereo broadcast by demodulating the sum and difference audio terms and then matrixing those two terms in order to yield the fundamental left and right audio signals L and R. The same receiver will respond to a monaural FM broadcast by reproducing the same monaural audio signal in both of its output channels. On the other hand, a monaural FM receiver will respond to the two channel broadcast stereo signal by deriving only the sum term (L+R) as represented in equation (1) and reproducing an audio signal that represents the monaural program. The two-channel signal thus is fully compatible with the monaural signal so that a receiver properly designed for one also will receive the other. Further detailed discussion of the foregoing two-channel transmission system and exemplary disclosures of transmitters and receivers for use therewith will be found in U.S. Pat. Nos. 3,257,511-Adler et al.; 3,257,512-Eilers; 3,129,288-DeVries and 3,151,218-Dias et al, all assigned to the same assignee as the present application.
In the last few years, interest has been evident in tape-recording systems wherein a four-channel stereo signal is recorded on magnetic tape. Four different audio signals are individually recorded on four respective different tracks along the tape. The four different audio signals represent sources respectively located at the left-front, right-front, left-rear and right-rear of an originating point. By using four different pick-up and amplification systems together with four separate loudspeakers similarly distributed around a listening point, four-channel reproduction is obtained.
The advent of four-channel stereo recording and reproduction has naturally led to consideration of the desirability of transmitting and receiving four-channel stereo signals by radio. Because two-channel stereo is now being broadcast by many FM transmitting stations, attention has been directed particularly to the possibility of utilizing broadcast stations in that category of service for the transmission of four-channel stereo in addition to, or instead of, the transmission of two-channel stereo or monaural signals. To accomplish this requires the development of a different overall transmission signal in order to accommodate the additional information components necessary to convey four separate channels. At the same time, it is desirable that any four-channel approach be fully compatible both with two-channel stereo and monaural, so that receiver obsolescence is avoided.
It is also desirable, from the standpoint of broadcast station economics, that a commercial four-channel stereo system provide for an SCA (Subsidiary Communications Authorization) channel.